It is known to produce straight bevel gears, as well as skew bevel gears, face couplings and splined parts, by providing a pair of inclined rotary cutting tools whose rotating cutting blades effectively interlock to simultaneously cut the same tooth space on a workpiece. Examples of this type of machining can be seen, for example, in U.S. Pat. No. 2,586,451 to Wildhaber; U.S. Pat. Nos. 2,567,273 and 2,775,921 to Carlsen; U.S. Pat. No. 2,947,062 to Spear or in the company brochure “Number 102 Straight Bevel Coniflex® Generator” published by The Gleason Works.
Straight bevel gears may be formed by a non-generating process where the inclined tools are plunged into the workpiece to form a tooth slot with the profile surface of the tooth being of the same form as that of the blade cutting edge. Alternatively, tooth surfaces may be generated wherein the inclined tools are carried on a machine cradle which rolls the tools together with the workpiece via a generating roll motion to form a generated profile surface on the workpiece. In either instance, the tools may also include cutting edges that are disposed at a slight angle (e.g. 3°) to the plane of cutter rotation. Such an angled cutting edge, in conjunction with the inclination of the tools, removes more material at the ends of a tooth slot thereby resulting in lengthwise curvature of the tooth surface (i.e. lengthwise ease-off) for tooth bearing localization.
Bevel and hypoid gears can be cut in a single indexing process (face milling) or in a continuous indexing process (face hobbing). A basic cutting setup in the generating or cradle plane will put the center of the cutter head in a position which is away from the generating gear center (cradle axis) by the amount known as the radial distance. The silhouette of the cutter blades represents one tooth of the generating gear while the cutter rotates. Common face cutters for bevel gear cutting have several blade groups with each group having between one and four blades. Most common are alternating (completing) cutters with one outside and one inside blade. Peripheral cutter heads for the manufacture of straight bevel gears according to the above-described interlocking cutters method use only one kind of blades (e.g. outside blades) which have been used on conventional mechanical machines in the past.
On modern CNC machines, such as those machines known as 6-axis or free-form machines and disclosed by, among others, U.S. Pat. No. 6,715,566 the disclosure of which is hereby incorporated by reference, only one cutter from the above-discussed interlocking pair of cutters is used to cut a first tooth flank in a lower cutting position and, with the same cutter, also cut a second tooth flank in an upper cutting position in a single indexing process as disclosed in, for example, U.S. Pat. No. 7,364,391 the disclosure of which is hereby incorporated by reference. The cutting of the first flank faces the problem that material has to be removed not only on the cutting edge but also on the clearance edge of the blade. The result is high part temperature, poor cutting performance and low tool life. In a combined process, of vector feed and rolling, the clearance side of the blades during the first slot cutting can be moved away from the material which protects the clearance side of the cutting edge. However, a vector feed in straight bevel gear cutting has to use a very steep angle (only few degrees away from the blades center line) which makes the blade tip subject of severe chip removing loads. This leads to early failure of the blade tips and therefore results in low tool life.
Similar conditions occur if in a face cutting process only one kind of blades is used (e.g. inside blades only or full profile blades). The cutting blades will only be optimal on one side for high amounts of chip removal, which increases part temperature and reduces tool life.